The small white gear in front is attached to the motor, which will be mounted to the bottom flange (originally was planned to be embedded in the copper block). The bottom ribbed half of this piece, aka the coliseum, is custom-made to fit the motor gear.
Not shown in these images are the particle compartments planned for the top half of the coliseum. We will have 4 rows of holes with diameter 0.8 mm to hold our mm-sized ice and dust samples.